Method and apparatus for storing and depleting energy

ABSTRACT

A method to control storage into and depletion from multiple energy storage devices. The method enables an operative connection between the energy storage devices and respective power converters. The energy storage devices are connectable across respective first terminals of the power converters. At the second terminals of the power converter, a common reference is set which may be a current reference or a voltage reference. An energy storage fraction is determined respectively for the energy storage devices. A voltage conversion ratio is maintained individually based on the energy storage fraction. The energy storage devices are stored individually with multiple variable rates of energy storage through the first terminals. The energy storage is complete for the energy storage devices substantially at a common end time responsive to the common reference.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/803,212, filed Mar. 14, 2013, which is hereby incorporated byreference in its entirety.

BACKGROUND 1. Technical Field

Embodiments herein relate to storage of electrical energy (e.g., in abank of multiple batteries) from electrical power sources, and powerdelivery (e.g., from the bank of multiple batteries) to an electricalnetwork.

2. Description of Related Art

Load balancing of electrical power refers to various techniques to storeexcess electrical power during low demand periods for subsequent releasewhen demand for electrical power increases. Storage of electrical energyfrom the electrical network may be performed within and/or outside theelectrical network. For example, the storage of electrical energy mayinvolve the customer and/or be performed on customer premises. Forexample, a storage electrical heater stores thermal energy during theevening, or at night when electricity is available at lower cost, andreleases the heat during the day. Replenishing energy during off peaktimes may require incentives for consumers to participate, usually byoffering cheaper rates for off peak electricity.

Load balancing may include customer owned energy storage/deliverysystems operating at times independently from the electrical networkand/or working in concert with the electrical network. Customer ownedenergy storage/delivery systems may include various energy sourcesincluding wind turbines, photovoltaic arrays, and/or fuel cells whichmay be independent and/or integrated with the battery storage. Customerowned energy storage/delivery systems may also be utilized to sell anddeliver electricity back to the electrical network during peak demand onthe electrical network.

BRIEF SUMMARY

According to features of the embodiments, various methods are providedto control storage into and depletion from multiple energy storagedevices. Energy storage devices are operatively connected to respectivepower converters. The energy storage devices are connectable acrossrespective first terminals of the power converters. At the secondterminals of the power converter, a common reference is set which may bea current reference or a voltage reference. An energy storage fractionis determined respectively for the energy storage devices. A voltageconversion ratio of the power converter is maintained individually basedon the energy storage fraction. Energy is stored individually withmultiple variable rates of energy storage through the first terminals.The energy storage is complete for the energy storage devicessubstantially at a common end time responsive to the common reference.Energy from the energy storage devices is depleted individually withmultiple variable energy depletion rates through the first terminals.The depleted energy is complete for the energy storage devicessubstantially at a common end time which is responsive to the commonreference. The energy storage devices may include energy convertersbetween electrical energy and at least one other form of energy.

According to features of the embodiments, various methods are providedfor control of charging and discharging of multiple batteries. Thebatteries are connectable across respective first terminals of the powerconverters. At the second terminals of the power converter a commonreference may be a current reference or a voltage reference. A batterycharge fraction may be determined respectively for the batteries. Avoltage conversion ratio, of the power converter, may be individuallymaintained based on the battery charge fraction. The batteries may becharged individually with multiple variable charging powers through thefirst terminals. The charging may be complete for the batteriessubstantially at a common end time responsive to the common reference.The batteries may be discharged individually with multiple variabledischarging powers through the first terminals. The discharging may becomplete for the batteries substantially at a common end time responsiveto the common reference.

While charging, the voltage conversion ratio may be maintainedsubstantially proportional to the amount of additional charge that maybe stored in the respective batteries. While discharging, the voltageconversion ratio may be maintained substantially proportional to theremaining available charge in the respective batteries.

The method further enables connection of the DC terminals of an AC/DCinverter to the second terminals and enables connection of the ACterminals of the AC/DC inverter to an AC electrical network source ofpower. The AC/DC inverter may be configured to set the common referencethrough the DC terminals. A central controller may be attached to theAC/DC inverter to provide a value for the common reference. The commonreference may be a current reference or a voltage reference.

With the second terminals of the power converters serially connected,the common reference may be set as a current reference. With the secondterminals of the power converters parallel connected, the commonreference may be set as a voltage reference.

The batteries may form a first bank of multiple batteries and a secondbank of multiple batteries like the first bank may be located indifferent geographic location from the first bank. The first and thesecond bank may share the common reference and consequently share samecommon end times for charging and discharging.

According to features of the embodiments, various systems may beprovided which control charge and discharge of multiple batteriesincluding multiple power converters each including first terminals andsecond terminals. The first terminals are connectable to the batteriesand the second terminals are connectable to a direct current (DC) sourceof power. At the second terminals of the power converter a commonreference may be set either a current reference or a voltage reference.Each of the power converters includes a controller operatively attachedto the power converter. The controller includes a fuel gauge operativelyattached to the respective battery. The fuel gauge with the controllerare operable to determine a battery charge fraction of the battery. Thecontroller may be operable to maintain a voltage conversion ratio basedon the battery charge fraction. The batteries are fully chargedsubstantially at a common end time responsive to the common reference.The batteries are fully discharged substantially at a common end timeresponsive to the common reference.

The common reference may be set as a current reference by a serialconnection of the second terminals of the power converters. The commonreference may be set as a voltage reference by a parallel connected ofthe second terminals of the power converters.

An AC/DC inverter includes AC terminals and DC terminals has the ACterminals connectable to an AC electrical network source of power. TheDC terminals may be operatively attached to the second terminals. TheAC/DC inverter may include a control portion configured to set thecommon reference through the DC terminals.

A central controller may be operatively attachable to the AC/DC inverterto provide a value for the common reference.

A first multiple of batteries form a first bank and a second multiple ofbatteries form a second bank like the first bank. The first bank andsecond bank may not be collocated, while sharing the common reference.The first and the second bank share the common end times for chargingand discharging responsive to the common reference.

Various systems may be provided or controlling energy storage into andenergy depletion from multiple energy storage devices. Multiple powerconverters include first terminals and second terminals and the energystorage devices are connectable across respective first terminals of thepower converters. A common reference may be set at the second terminals,wherein the common reference may be current reference or a voltagereference. An energy gauge may be configured to determine an energystorage fraction respectively for the energy storage devices. The powerconverters maintain individually a voltage conversion ratio, based onsaid energy storage fraction. Energy may be stored in the energy storagedevices individually with a multiple variable rates of energy storagethrough the first terminals. The energy storage may be complete for theenergy storage devices substantially at a common end time responsive tothe common reference.

In some variations, energy may be depleted from the energy storagedevices jointly and/or individually. Where the energy may be depletedusing multiple variable energy depletion rates and/or a constant rate.The energy may also be depleted through one or more terminals such asthe first terminals. The energy depletion may also be configured tooccur through one power converters. The rates of energy depletionthrough any one terminal and/or power converter may be independentand/or dependent of the variable rates of energy depletion through anyother terminal and/or power converters. In some embodiments, the energydepletion for the energy storage devices is substantially at a commonend time. The common end time may be achieved through suitablemechanisms such as using a common reference (e.g., a digital and/oranalog reference). For example, a first rate of depletion of a firstenergy storage device may be accelerated as compared to a second rate ofdepletion of a second energy storage device so that all of the energystorage devices reach a depleted state substantially at a common endtime. This may be accomplished via one or more local and/or remotecontrollers which may be coupled to one or more of the energy storagedevices and/or configured to periodically and/or continually monitor theenergy storage devices in order to control the depletion of thesedevices. Other feedback control mechanisms may also be utilized. Thesemay be controlled locally and/or networked to a remote location andcontrolled in conjunction with other energy storage devices at variouslocations. The foregoing and/or other aspects will become apparent fromthe following detailed description when considered in conjunction withthe accompanying drawing figures.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments are herein described, by way of example only, with referenceto the accompanying drawings, wherein:

FIG. 1 shows a hybrid power generating system, according to a feature ofthe embodiments.

FIG. 2 shows an implementation of an energy bank shown in FIG. 1,according to a feature of the embodiments.

FIG. 3 shows more details of the direct current (DC) to DC convertershown in FIG. 2, according to a feature of the embodiments.

FIG. 3a illustrates a buck plus boost converter according to a featureof the embodiments.

FIG. 4 shows a method applied to the implementation of the energy bankshown in FIG. 2, according to a feature of the present inventionembodiments.

FIGS. 5-6 show other implementations of the energy bank shown in FIG. 1,according to various embodiments.

FIG. 7 shows a method for energy storage according to one or moreembodiments.

DETAILED DESCRIPTION

Reference will now be made in detail to features of the embodiments,examples of which are illustrated in the accompanying drawings, whereinlike reference numerals refer to the like elements throughout. Thefeatures are described below to explain the embodiments by referring tothe figures.

Before explaining features of the embodiments in detail, it may beunderstood that the embodiments are not limited in its application tothe details of design and the arrangement of the components set forth inthe following description or illustrated in the drawings. Theembodiments are capable of other features or of being practiced orcarried out in various ways. Also, it is to be understood that thephraseology and terminology employed herein is for the purpose ofdescription and should not be regarded as limiting.

Aspects of the embodiments are directed to charging and discharging ofenergy storage devices. The storage of energy and depletion of energy oftwo or more energy storage devices takes into account the amount ofstored energy relative to the capacity of energy storage in the energystorage device. According to an aspect of the embodiments, the energystorage devices may be batteries and respective end times of chargingand discharging of two or more of the batteries may be remotely setresponsive to a common voltage or current reference. All the batteriesmay be fully charged or fully emptied at the same time which may becontrollable remotely using the common voltage or current reference. Theenergy storage and energy depletion may thereby be balanced to preventsome of the energy storage devices to be empty when other energy storagedevices still contain energy.

Aspects of the embodiments are directed to controlling energycharge/discharge between any number of energy storage devices, e.g.batteries. Balance and control of charge/discharge of batteries may usedifferent types of batteries, different capacitors and/or other types ofenergy storage devices in the same system.

It should be noted that the embodiments, by non-limiting example,alternatively be configured to include different energy sources such aswind turbines, hydro turbines, fuel cells; mechanical energy storagesuch as a flywheel, gas pressure, spring compression, pumping waterand/or lifting mass against gravity; and chemical energy storage such asbattery and fuel cell.

The terms “alternating current (AC) network”, “AC power supply” and“power network” as used herein are used interchangeably and refer to anAC power source. The AC power source typically supplies power todomestic, industrial, infrastructure, or facility-based processesseparately thereto or in addition to an AC electrical network providedfrom an electricity utility company. The AC power source may be derivedfrom an AC generator or the output of a direct current (DC) to ACinverter. A DC input of the DC to AC inverter may transfer electricalenergy sourced from for instance a photovoltaic array, fuel cells,batteries and/or DC generator.

Referring now to the drawings, FIG. 1 shows a power generation andstorage system 10, according to a feature of the embodiments. Analternating current (AC) electrical network 12 may be connected to amultiple electrical power sources generally AC generation units 16 and anumber of energy banks 14 connected to AC electrical network 12 atrespective nodes A and B. AC electrical network 12 may be shown in FIG.1 as a single phase electrical network but alternative embodiments ofthe embodiments may be configured with a 3 phase electrical network. ACelectrical network 12 may be part of a public AC electrical network or aprivate AC electrical network. AC generation units 16 may include ACpower produced from wind turbines, hydro turbines, fuel cells,super-conducting flywheel, and capacitors, and mechanical devicesincluding conventional and variable speed diesel engines, Stirlingengines, gas turbines, and/or photovoltaic panel arrays. The power maybe produced by AC generators and/or DC/AC inverters or a combinationthereof such as combined AC outputs of micro-inverters. Energy banks 14in general may provide the capability to store excess AC power fromelectrical network 12 and/or store AC power during periods of time whenthe cost of producing AC power may be relatively inexpensive. Energybanks 14 in general may provide the capability to provide previouslystored power onto electrical network 12 when demanded.

One or more local and/or remote controllers such as central controller110 may be operatively attached to energy banks 14 to determine how andwhen energy banks 14 may provide or receive AC power to/from electricalnetwork 12. Central controller 110 may monitor the status of electricalnetwork 12 via sensor 18 with respect to voltage, current, phase angle,power factor, real power, apparent power and/or reactive power. Byand/or, it is meant that these items may be used in any combinationand/or subcombination.

An energy bank 14 may be physically located with an AC generation unit16 and both energy bank 14 and AC generation unit 16 may provide thecapability to source/sink AC power onto/from a local electrical networkassociated with the AC generation unit 16. The local electrical networkmay be loaded with the power demands of a factory for example. The localelectrical network may disconnect from electrical network 12 and relyentirely on power delivered from the AC generation unit 16 and/or theenergy bank 14. The energy bank 14 may be comprised of one or moresubbanks 14 a, 14 b, etc., which may or may not be collocated.

Reference is now made to FIG. 2, which shows an exemplary implementation14 a of energy bank 14, according to embodiments. Multiple energystorage devices, e.g. batteries 20 may be connected to first terminals260 of respective power converter modules 202. At first terminals 260(reference denoted for one of batteries 20), voltage across battery 20while charging may be denoted as V_(c) and voltage across battery 20while discharging may be denoted V_(bat). In general, the batteryvoltages V_(bat) for batteries 20 may be different, as are chargingvoltages V_(c) for batteries 20. Second terminals 262 of powerconverters 202 are connected together in a serial string 350.Alternatively, a parallel connection may also be utilized. Each energystorage device may include one or more batteries and or battery cellsconnected in series, parallel or combinations thereof. Further, eachenergy storage device may be variously configured to include otherenergy storage devices, e.g., fuel cell, capacitor, etc., in addition toand/or instead of the described battery. Further, control systems may beused in each energy source individually, and/or across several energysources, and/or distributed locally and/or remotely.

Power converter modules 202 may include bi-directional direct current(DC) to DC converters. The DC voltage across second terminals 262 of onepower converter module 202 may be denoted as voltage V. Power convertermodules 202 may convert (to a high efficiency) power V×I_(Ref) fromsecond terminals 262 to a power V_(C)×I_(Bat) at first terminals 260used to charge a battery 20 on first terminals 260 or to dischargebattery 20 by converting power from battery 20 (I_(Bat)×V_(Bat)) onfirst terminals 260 to a power V×I_(Ref) on second terminals 262. Thepower V×I_(Ref) on second terminals 262 used to discharge a battery 20provides (to a high efficiency) power onto electrical network 12 viaDC/AC inverter 200. Serial string 350 may be connected in parallelacross DC terminals W and X of inverter 200. AC terminals Y and Z ofinverter 200 connect to electrical network 12 at nodes A and B.

Inverter 200 may be configured to be bi-directional, to convertalternating current (AC) power on AC terminals Y and Z to a DC power onDC terminals W and X for charging or convert DC power on DC terminals Wand X to an AC power on terminals Y and Z for discharging batteries 20and providing the power to electrical network 12. Inverter 200 may becontrolled by and/or monitored by central controller 110, for instanceby power line communications or by wireless communications.

In alternative embodiments of the embodiments, power converter module202 may include a DC/AC bi-directional micro-inverter and then inverter200 may not used. Where each power converter module 202 may be abi-directional switching micro-inverter, string 350 connects directlyacross AC electrical network 12 at nodes A and B. String 350 connectedacross AC electrical network 12 at nodes A and B allows conversion of aportion of AC 12 to a DC voltage V_(C) which may be used to charge arespective battery 20. Conversely the DC voltage V_(Bat) of a battery 20may be converted to a portion of AC 12, thereby discharging a respectivebattery 20.

In various embodiments of 14 a, a first multiple of batteries 20 mayform a first bank of batteries and a second multiple of batteries 20 mayform a second bank that is similar or different from the first bank. Thefirst bank and second bank may not be collocated, but may share thecommon reference Iref.

Reference is now made to FIG. 3 which shows details of the directcurrent (DC) to DC converter 202 shown in FIG. 1, according toembodiments. Power converter module 202 may include a fuel gauge 302attached to sensor 310 which may monitor the voltage and current on boththe first terminals 260 of converter circuit 300. Fuel gauge 302 may becontrolled by one or more controllers 306 (e.g., a microprocessor, ASIC,FPGA, linear feedback control system, CPU, logic, firmware, and/or othersuitable device) according to an algorithm (e.g., an algorithm stored inmemory 304), may further monitor; the temperature, internal resistance,state of charge and/or state of discharge of each respective battery 20.Batteries 20 may also be different types of batteries with respectivebattery characteristics stored in memory 304. The controller may controlpower converter 300 to charge and discharge battery 20 based on anysuitable parameters such as sensor data, the stored batterycharacteristics, and/or dynamic feedback mechanisms.

Power converter circuit 300 may be variously configured. For example, itmay be a DC-DC converter, and DC-AC converter, an AC-AC converter and/oran AC-DC converter. In some embodiments, power converter 300 is a DC-DCconverter configured to include a buck stage followed by a boost stageor a boost stage followed by a buck stage. Further details of powerconverter circuit 300 may be now made by reference also to FIG. 3a whichillustrates a buck plus boost converter 300 according to a feature ofthe embodiments. Buck plus boost converter 300 has a buck circuit 320.An inductor 328 and common rail 329 connect buck circuit 320 to boostcircuit 322. Buck circuit 320 has a low side buck MOSFET GA, connectedbetween common rail 329 and one side of inductor 328 and one side of ahigh side buck MOSFET GC, the other side of a high side buck MOSFET GCconnects to one terminal of first terminals 260. A capacitor C₁ may beshunt connected across first terminals 260. Boost circuit 322 has a lowside boost MOSFET GB, connected between common rail 329 and the otherside of inductor 328 and one side of a high side boost MOSFET GD. Theother side of a high side boost MOSFET GD connects to one terminal ofsecond terminals 262. A capacitor C₂ may be shunt connected acrosssecond terminals 262. The symmetry of converter 300 may be such thatdepending on how MOSFETs GA, GB, GC and GD are driven and controlled,converter 300 may be a buck stage followed by a boost stage or a booststage followed by a buck stage. In other variations, the buck circuitand boost circuit each have respective inductors in place of commoninductor 328. Further variations may configure GA, GB, GC, and GD asswitches (e.g., semiconductor switches). In various examples, switchesGA, GB, GC, and GD are controlled by control circuitry that may includeone or more programmable pulse width modulators, controllers, and otherlogic. In various embodiments, the control circuitry may be one or moreseparate devices, disposed locally and/or remotely, and/or may beintegrated within controller 306.

Reference is now also made to FIG. 4 which shows a method 401, accordingto a feature of the embodiments. In step 403, batteries 20 may beconnected to first terminals 260 of respective converter modules 202.Second terminals 262 may be connected together in series to form string350. In other embodiments, step 403 may include the second powerinterfaces 262 of the power converter modules 202 being connectedtogether in parallel as further described herein.

In step 405 current reference I_(Ref), that may be the current in serialstring 350 may be set to a programmed current by inverter 220. Thecurrent value of current reference I_(Ref) may be programmed within thecontrol circuitry of inverter 220 via central controller 110. In step407, a battery charge fraction may be determined for each battery 20 byfuel gauge 302, microprocessor 306 and stored in memory 304. The batterycharge fraction may be defined herein by the fraction: the charge storedin battery 20 divided by the charge capacity of battery 20. The chargecapacity of a battery in the first case when battery 20 is new is knownand stored in memory 304 of fuel gauge 302. Monitoring by fuel gauge 302in subsequent charging and discharging cycles of batteries 20, may allowthe charge capacity and/or battery charge fraction to be updated in step407 as well as providing further information for the setting currentreference I_(Ref) in step 405 also.

In alternate embodiments, Battery charge fraction (BC), may bedetermined, for example, by integrating the discharge and charge current(IBat) over a duration to calculate the change of charge in the battery.The change in charge may then be subtracted from the charge capacity todetermine remaining charge stored in the battery. Step 407 may furtherinclude updating the setting of current reference, I_(Ref), based on thecharge fraction and/or charge capacity.

In step 409, each battery 20 may be charged individually assuming thateach power converter 300 may be substantially 100% efficient, with acharging power (P_(charge)):

P_(charge) = E × C × (1 − BC) = V × I_(Ref) $C = \frac{V_{C}}{t_{c}}$where

-   -   C=charge factor of the battery 20 (volts per hours)    -   V_(C)=voltage at first terminals 260.    -   t_(c)=charging time.    -   V=voltage at second terminals 262.    -   E=charge storage capacity of a battery 20 (ampere-hours).    -   BC=battery charge fraction of the battery 20.    -   I_(Ref)=reference current through second terminals 262 of power        converter 300    -   I_(Bat)=current charging battery 20.

Re-arranging and solving for charging time t_(c)

$t_{c} = \frac{E \times V_{C} \times \left( {1 - {BC}} \right)}{V \times I_{Ref}}$

In the equation for t_(c) above it can be seen that according to afeature of the embodiments, if converters 300 maintain individually avoltage conversion ratio (V_(c)/V) proportional to the reciprocal ofE·(1−BC), the amount of additional charge that may be stored in battery20, then t_(c) may be fully determined by the current reference I_(Ref).Consequently, for multiple batteries, charging may be completed forbatteries 20 substantially at a common end time t_(c) which may beresponsive to current reference I_(Ref).

In step 411, each battery 20 may be discharged individually assumingthat each converter 300 may be substantially 100% efficient, with apower (P_(discharge)):

P_(discharge) = E × D × BC = V I_(Ref) $D = \frac{V_{Bat}}{t_{d}}$where

-   -   D=discharge factor of the battery 20.    -   V_(Bat)=Voltage of battery 20.    -   t_(d)=discharging time    -   E=charge storage capacity of a battery 20 (ampere-hour)    -   BC=battery charge fraction of battery 20.    -   V=voltage between second terminals 262 of power converter 202.    -   I_(ref)=reference current through second terminals 262 of power        converter 202.

Re-arranging and solving for charging time t_(d)

$t_{d} = \frac{E \times V_{Bat} \times {BC}}{V \times I_{Ref}}$

In the equation for t_(d) above, it can be seen that according to afeature of the embodiments, if converters 202 maintain individually avoltage conversion ratio (V_(bat)/V) proportional to the reciprocal ofE·BC, the remaining available charge in battery 20, then t_(d) may befully determined by the current reference I_(Ref). Consequently, formultiple batteries, discharging may be completed for batteries 20substantially at a common end time td which may be responsive to currentreference I_(Ref).

In step 413, by virtue of each battery 20 being charged or dischargedindividually at respective first terminals 260 according to steps 409 or411 respectively, batteries 20 are substantially fully charged orsubstantially fully discharged at substantially the same time.

A data history may be logged in memory 304 and include type of battery20, the state of charge or discharge of respective batteries 20, alongwith a data of the present state of charge or discharge of respectivebatteries 20. Storage of the data history with the data for a battery 20which may include an charge storage capacity, a charge stored, a chargefactor, a battery charge fraction, a battery charge and a batterydischarge factor of respective batteries 20. The charge storage capacityof a battery 20 may include the usable capacity of charge available frombattery 20, the portion of battery 20 which may be empty andrechargeable and an unusable capacity which can no longer be rechargedbecause of deterioration of battery 20 with usage over time. The batterycharge fraction may be defined by the energy stored in battery 20divided by the energy capacity of battery 20

In general, the charge or discharge current I_(Bat) depends on amount oftime to charge or discharge the batteries 20. The amount of time tocharge or discharge the batteries 20 may be based on the common currentreference (I_(Ref)) and/or an end time of charging or discharge forexample which may be at six am in the morning for example. Depending onthe time from when batteries 20 are desired to be charged or discharge,the amount of time to charge or discharge the batteries 20 may bedetermined based on the common current reference (I_(Ref)). If the timefrom when batteries 20 are desired to be charged or discharge is nine inthe evening, then the amount of time to charge or discharge thebatteries 20 is 9 hours. Alternatively a shorter amount of time forcharging or discharging with the amount of time to charge or dischargethe batteries 20 may be selected based on the common current reference(I_(Ref)). In general, the greater the level of the common currentreference (I_(Ref)) means a shorter period of time to charge ordischarge batteries 20. The initiation of the charging or discharging ofbatteries 20 may be controlled by central controller 110 to inverter 200and converter 202. The level of the current reference (I_(Ref)) may beset by central controller 110 and controlled by inverter 200.

Reference is now made to FIG. 5 which shows an energy bankimplementation 14 b, according to a feature of the embodiments. Multiplebatteries 20 are connected to first terminals 260 of respective powerconverters 202. The second terminals 262 of power converters 202 areconnected in parallel to give a parallel connection. Where converters202 are bidirectional DC to AC converters, the parallel connection maybe made directly across AC 12 at nodes A and B. Where power convertermodules 202 include bidirectional DC to DC converters 300, the parallelconnection may be made directly across terminals W and X of abidirectional inverter 200 and terminals Y and Z of inverter 200 areconnected across AC 12 at nodes A and B.

The parallel connection gives serves as a common voltage reference(V_(Ref)) for converters 202. Power converters 202 may convert powerfrom V_(Ref)×I_(C) to a power V_(C)×I_(Bat) used to charge a battery 20on the first terminals 260 or convert power from battery 20(I_(Bat)×V_(Bat)) on the first terminals 260 to a power I_(C)×V_(Ref) onthe second terminals 262. The power I_(C)×V_(Ref) on the secondterminals 262 used to discharge a battery 20 provides power ontoelectrical network 12 via inverter 200. Both inverter 200 and converters202 include all of the features described in the description of FIG. 2above.

Reference is now made again to method 401 applied to energy bankimplementation 14 b shown in FIG. 5, according to a feature of theembodiments.

Method 401 shows batteries 20 in an energy bank 14 b may be charged ordischarged according to a common voltage reference (V_(Ref)). By way ofexample, where five batteries 20 are to be charged, it is assumed thateach of the five batteries 20 are of the same type with an open circuitvoltage of 25 volts and charge/discharge rating of 10 ampere hours, AC12 and therefore the common voltage reference (V_(Ref)) may be 240 voltsroot mean square (RMS) or 240 volts DC on each converter 202.

In step 403, batteries 20 are connected to first terminals 260 ofrespective converters 202. Second terminals 262 are connected togetherin parallel to the terminals W and X of inverter 200.

In step 405 voltage reference V_(Ref) may be set to be constant byinverter 220 via central controller 110 for the purpose of chargingbatteries 20. Consequently as a result of voltage reference V_(Ref) setconstant, each current (I_(C)) through second terminals 262 of eachconverter 202 will be different because of respective usable capacity ofenergy available from a battery 20 at any point in time will bedifferent. In step 407, a battery charge fraction may be determined foreach battery 20. Monitoring by fuel gauge 302 in subsequent charging anddischarging cycles of batteries 20, allows the battery charge fractionto be determined in step 407 as well as providing further informationfor the setting voltage reference V_(Ref) in step 405 also.

In step 409, each battery 20 may be charged individually, assuming thateach converter 202 may be substantially 100% efficient, with a power(P_(charge)). The charging powers (P_(charge)) on each of firstterminals 260 according to the equation above for P_(charge) aretherefore responsive to the remaining energy desired to fully charge therespective battery 20 and responsive to voltage reference V_(Ref).

In step 411, each battery 20 may be discharged individually assumingthat each converter 202 may be substantially 100% efficient, with apower (P_(discharge)). The discharging powers (P_(discharge)) on each offirst terminals 260 according to the equation above for P_(discharge),are therefore responsive to the remaining energy desired to fullydischarge the respective battery 20 and responsive to voltage referenceV_(Ref). The charging powers (P_(charge)) on each of first terminals 260according to the equation above for P_(charge) are therefore responsiveto the remaining energy desired to fully charge the respective battery20 and responsive to voltage reference V_(Ref).

In step 413, by virtue of each battery 20 being charged or dischargedindividually at respective first terminals 260 according to steps 409 or411 respectively, batteries 20 are substantially fully charged orsubstantially fully discharged at substantially the same time.

The method 401 and system components described above balance and controlcharge/discharge of batteries 20 which can use different types ofbatteries with different capacities along with other types of energystorage in the same system. By using method 401, the system 10 controlsthe energy storage between and in energy banks 14. Within an energy bank14, the energy may be spread in a balanced way that prevents part of anenergy bank 14 to be partly empty when the other part still containsenergy. In a similar way between energy banks 14, the energy may bespread in a balanced way that prevents one or more energy banks 14 to bepartly empty when one or more energy banks 14 still contains energy.Therefore, batteries 14 are balanced so that if one battery 20 may beconnected, after number of charges and discharges the battery 20 willget to the same charge fraction as the others. The situation that abattery 20 will be empty before another battery 20 or that a battery 20may be full or empty before all the other batteries 20 are discharged orcharged may be also avoided.

In still further embodiments, Battery charge fraction (BC), may bedetermined, for example, by integrating the discharge and charge current(IBat) over a duration to calculate the change of charge in the battery.The change in charge may then be subtracted from the charge capacity todetermine remaining charge stored in the battery. Step 407 may furtherinclude updating the setting of current reference, I_(Ref), based on thecharge fraction and/or charge capacity. In step 409, each battery 20 maybe charged individually and independently based on the current reference(IRef) set in step 405, and based on controlling the voltage conversionratio (r=Vc/V) and charging time period (t_(c)). For example, the amountof charge (Qc) put into the battery is given by:Qc=IBat*tc  (eq. 1).

The charging current (IBat) can be determined from the efficiency of thepower converter (e), and the input reference voltage as follows:Pin=(V*IRef)=(e*Pcharge)=e*(Vc*IBat)   (eq.2).

Solving equation 2 for IBat results in:IBat=(V*IRef)/(Vc*e)=(1/e)*(V/Vc)*IRef  (eq. 3).

Substituting IBat from equation 3 into equation 1 and replacing (Vc/V)with the voltage conversion ratio (r) controlled by controller 306and/or controller 110 results in the following relationship.Q=(1/e)*(1/r)*IRef*tc  (eq. 4).

Thus, in various embodiments, the amount of charge (Q) into the batterymay be controlled based on controlling (e.g., with controller 306 and/orcontroller 110) the voltage conversion ratio (r), the current reference(IRef) at the second power interface 262, and the charging time period(tc). The voltage conversion ratio may, for example, be controlled in abuck plus boost converter by varying the duty cycle of the switching inthe converter.

In certain variations, for the common reference IRef, step 409 mayinclude charging each battery 20 independently by controlling conversionratio (r) and the charging time period (tc) to a percentage (p) of theremaining capacity (Qc) in the battery, given by:Qc=E*(1−BC)=(1/e)*(1/r)*IRef*tc  (eq. 5),where:

-   -   E=charge storage capacity of battery 20 (ampere-hours).    -   BC=battery charge fraction of battery 20.

Various embodiments include each power converter 300 maintaining (e.g.,with controller 306 and/or controller 110) individually andindependently a respective voltage conversion ratio (r) proportional tothe reciprocal of the remaining capacity (Qc) in the respective battery20:r=1/(p*Qc)=1/(p*E*(1−BC))  (eq. 6),where:

-   -   p=the percentage of remaining capacity Qc to charge over tc.

By incorporating equation 6 into equation 5, we find that charge onlydepends on IRef in the following manner:E*(1−BC)=(1/e)*(p*E*(1−BC))*IRef*tc  (eq. 7),which simplifies to:tc=(p/e)*(1/IRef)  (eq. 8).

Thus, in this embodiment, the charge time (tc) for each battery 20 maybe inversely proportional to reference current IRef in response to eachpower converter 300 being controlled individually an independently suchthat voltage conversion ratio of the respective power converter isinversely proportional the remaining capacity (Qc) of the respectivebattery 20. In various embodiments in which the percentage (p) andconverter efficiency (e) are the same or approximately the same acrossthe multiple converters module 202 in string 350, the respectivebatteries 20 will be charged to full capacity (E) in the same time tc,since all of the converters have the same reference IRef. Note that thisis the case regardless of whether each battery has the same remainingcapacity or a different remaining capacity compared to other batteries.

In these embodiments, the charge time may be the same regardless ofwhether each battery has the same remaining capacity or a differentremaining capacity compared to other batteries, because the chargingcurrent IBat for each battery is variable and controlled based on theindividual batteries remaining capacity.

Efficiency (e) may be estimated in some variations to be 100% incontrolling the conversion ratio r. In other variations, actualefficiency may be determined for each power converter module 202 andstored in memory 304, in controller 306 or in controller 110. Controller306 and controller 110 may control each power converter 300 based on theactual efficiency of each converter. Actual efficiency may be apredetermined value provided by a manufacture or may be determined,e.g., by testing. Actual efficiency may include multiple efficiencyvalues for each converter depending on operating conditions, and controlof each converter may be based on the multiple efficiency values atmultiple respective operating conditions (e.g., buck mode, boost mode,duty cycles, temperature, current, voltage, duty cycle, etc.) Operatingconditions may be measured values taken during charging and dischargingoperations.

In some variations, percentage (p) may be a value pre-set value fromzero to one (i.e., 0% to 100%). In other variations, percentage (p) maybe a value stored in each memory 304, or in a memory or register withincontroller 110. In certain variations, p may be set to have differentpredetermined values for one or more of the batteries 20 so that eachbattery is charged to predetermined different rates and levels.Percentage p may also be set to compensate for differences in converterefficiencies (e) (e.g., set each battery p/e to a common predeterminedvalue) and differences in battery conditions (e.g., environmentalconditions).

Reference is now made to FIG. 6 which shows an energy bankimplementation 14 c, according to an feature of the embodiments. Theenergy bank implementation 14 c may be the same as energy bankimplementation 14 b shown in FIG. 5 except electrical connections toterminals of batteries 20 are replaced with electrical connections toterminals of energy storage devices 21. Similarly batteries 20 shown inFIG. 2 may also be replaced with energy storage devices 21. Energystorage devices 21 may be elector-mechanical devices such as a DC motorand/or generator for example. The DC motor and/or generator when actingas a motor may be capable of converting electrical energy available onthe electrical connections of the motor to a mechanical energy. Themechanical energy may be used, for instance to pump water againstgravity to a holding pool. The water from the holding pool may bereleased later to produce electricity when the energy storage device 21serves as an electrical generator and/or turbine. The electricalgenerator and/or turbine, therefore converts the mechanical energy ofwater flow over the turbine to electricity, thereby depleting the storedenergy in the holding pool. Other electro-mechanical devices for energystorage and/or energy depletion may include springs, weights,compressors to compress gas into a sealed tank, cavities or sealedunderground caves for later release, fly wheels conventional andvariable speed diesel engines, Stirling engines, gas turbines, andmicro-turbines. Energy storage devices 21 may be electrochemical devicessuch as a fuel cell or a battery. Energy storage devices 21 may also beelectrostatic devices such as a capacitor. Energy storage devices 21 maybe a combined electro-thermal system which may use molten salt to storesolar power and then dispatch that power as desired. The electro-thermalsystem pumps molten salt through a tower heated by the sun's rays and/orheat which may be taken away from a photovoltaic array too. The pumpingof the molten salt may be via electricity from the photovoltaic arrayand/or a mains electricity electrical network. Insulated containersstore the hot salt solution and when desired, water may then be added tothe stored molten salt to create steam which may be fed to turbines togenerate electricity.

Various embodiments may intermix and combine the different types ofenergy storage devices within an energy bank 14. Different energy banks14 within system 10 may include different combinations of energy storagedevices. In the embodiments described above with energy storage devicesreplacing batteries 20, the above description of battery charge fraction(BC) with respect to batteries is equivalent to an energy storagefraction of the energy storage device. That is, energy storage fractionis the fraction of energy stored in the energy storage device divided bythe energy storage capacity of the energy storage device. Further,charging and discharging with respect to battery 20 is equivalent tostoring and depleting energy with respect to the energy storage device.

Reference is now made to FIG. 7 which shows a method 701 for energystorage, according to an feature of the embodiments. In the discussionof the method steps 703-713 of method 701 that follows, the term “energystorage” to an energy storage device 21 may be used. Steps 403-413 ofFIG. 4, may considered as analogous or equivalent in principle to steps703-713 where the energy storage fraction may be equivalent to a batterycharge fraction and storing and depletion of energy may be equivalent tocharging and discharging of batteries respectively.

In step 703, an operative connection between the energy storage devices21 and respective power converters 202 may be made. The operativeconnection allows energy conversion by energy storage devices 21 (forexample electrical energy to mechanical energy and vice versa) so thatelectrical energy may flow to and from converters 202.

In step 705 voltage reference V_(Ref) may be set constant (with respectto FIG. 6) or current reference I_(Ref) to be set constant (with respectto FIG. 2) by inverter 220 via central controller 110. Consequently as aresult of voltage reference V_(Ref) or I_(Ref) set constant, eachcurrent through second terminals 262 of each converter 202 will bedifferent because of respective usable capacity of energy available froman energy storage device 21 at any point in time will be different.

In step 707, an energy storage fraction may be determined for eachenergy storage device 21. The energy storage fraction may be energystored in an energy storage 21 divided by the energy capacity of anenergy storage 21. A circuitry like fuel gauge 302 and memory 304attached to microprocessor 306 may be used to provide information withrespect the energy storage capacity, monitoring and measuring of energystorage 21 and depletion of an energy storage 21. The circuitry may thencontrol buck+plus converter 300 by maintaining a voltage conversionratio of converter 300 based on the energy storage fraction.

In steps 709 and 711 for energy storage and energy depletionrespectively, steps 709 and 711 are applied to FIG. 2 with energystorage devices 21 instead of batteries 20. In step 709, the powerdesired for energy storage in energy storage 21 may be expressed by thesame equation used for the charging of batteries 20, described above. Instep 709 energy may be stored in each energy storage 21 individuallywith multiple variable rates of energy storage through respectiveconnections at first terminals 260.

Similarly, in step 711, the power desired for energy depletion fromenergy storage 21 may be expressed by the same equation used for thedischarging of batteries 20, described above. In step 711 energy may bedepleted from each energy storage 21 individually, with multiplevariable rates of energy depleted through respective connections atfirst terminals 260.

In step 713, by virtue of each energy storage device 21 having energystored or depleted individually at respective first terminals 260according to steps 709 or 711 respectively, energy storage devices 21have substantially a full amount of energy stored or substantially fullydepleted at substantially the same time.

Although selected features of the embodiments have been shown anddescribed, it is to be understood the embodiments are not limited to thedescribed features. Instead, it is to be appreciated that changes may bemade to these features without departing from the principles and spiritof the embodiments, the scope of which is defined by the claims and theequivalents thereof.

I claim:
 1. A method comprising: determining a corresponding chargefraction for each of a plurality of energy storage devices, wherein theplurality of energy storage devices are connected one-to-one to aplurality of power converters; and for each of the plurality of energystorage devices, independently maintaining, by a corresponding powerconverter of the plurality of power converters, a corresponding voltageconversion ratio such that the plurality of energy storage devices arecharged within a common charge time that is based on: the correspondingcharge fractions for all of the plurality of energy storage devices, andtransfer of power from an electrical network and to the plurality ofenergy storage devices.
 2. The method of claim 1, wherein, for each ofthe plurality of energy storage devices, the independently maintainingthe corresponding voltage conversion ratio comprises: updating, based oncorresponding charge capacities for all of the plurality of energystorage devices, one of a common current reference or a common voltagereference at terminals of the plurality of power converters, wherein theterminals are connected to an alternating-current-to-direct-currentinverter.
 3. The method of claim 2, wherein the common charge time isbased on the updated one of the common current reference or the commonvoltage reference.
 4. The method of claim 2, wherein the updatingcomprises: determining, by the plurality of power converters and via acentral controller, a second common current reference or a second commonvoltage reference; and setting direct-current terminals of thealternating-current-to-direct-current inverter to the second commonvoltage reference or the second common current reference.
 5. The methodof claim 2, wherein the updating comprises updating the common currentreference at serially connected terminals of the plurality of powerconverters that form a serial connection, wherein the serial connectionis connected to the alternating-current-to-direct-current inverter. 6.The method of claim 2, wherein the updating comprises updating thecommon voltage reference at the terminals of the plurality of powerconverters that are connected in parallel to form a parallel connection,wherein the parallel connection is connected to thealternating-current-to-direct-current inverter.
 7. The method of claim1, further comprising an action of: monitoring a plurality of fuelgauges respectively connected one-to-one to the plurality of energystorage devices, wherein the determining of the respective chargefraction for each of the plurality of energy storage devices is based onthe monitoring of the plurality of fuel gauges.
 8. A method comprising:determining a corresponding charge fraction for each of a plurality ofenergy storage devices, wherein the plurality of energy storage devicesare connected one-to-one to a plurality of power converters; and foreach of the plurality of energy storage devices, independentlymaintaining, by a corresponding power converter of the plurality ofpower converters, a corresponding voltage conversion ratio such that theplurality of energy storage devices are discharged within a commondischarge time that is based on: the corresponding charge fractions forall of the plurality of energy storage devices, and transfer of powerfrom the plurality of energy storage devices and to an electricalnetwork.
 9. The method of claim 8, wherein, for each of the plurality ofenergy storage devices, the independently maintaining of thecorresponding voltage conversion ratio comprises: updating, based oncorresponding charge capacities for all of the plurality of energystorage devices, one of a common current reference or a common voltagereference at terminals of the plurality of power converters, wherein theterminals are connected to a direct-current-to-alternating-currentinverter.
 10. The method of claim 9, wherein the common discharge timeis based on the updated one of the common current reference or thecommon voltage reference.
 11. The method of claim 9, wherein theupdating comprises: determining, by the plurality of power convertersand via a central controller, a second common current reference or asecond common voltage reference; and setting direct-current terminals ofthe direct-current-to-alternating-current inverter to the second commonvoltage reference or the second common current reference.
 12. The methodof claim 9, wherein the updating comprises updating the common currentreference at serially connected terminals of the plurality of powerconverters that form a serial connection, wherein the serial connectionis connected to the direct-current-to-alternating-current inverter. 13.The method of claim 9, wherein the updating comprises updating thecommon voltage reference at the terminals of the plurality of powerconverters that are connected in parallel to form a parallel connection,wherein the parallel connection is connected to thedirect-current-to-alternating-current inverter.
 14. The method of claim8, further comprising: monitoring a plurality of fuel gaugesrespectively connected one-to-one to each of the plurality of energystorage devices, wherein the determining of the respective chargefraction for each of the plurality of energy storage devices is based onthe monitoring of the plurality of fuel gauges.
 15. A system comprising:an energy storage device; fuel gauge circuitry configured to determine acharge fraction for the energy storage device; a power convertercomprising first terminals connected to the energy storage device; aninverter comprising: second terminals connected to the power converter,and third terminals connected to an electrical network; and a centralcontroller configured to adjust, based on the charge fraction for theenergy storage device determined by the fuel gauge circuitry, a voltageconversion ratio for the energy storage device such that the energystorage device completes charging or discharging at a substantiallycommon time as another energy storage device connected to the electricalnetwork.
 16. The system of claim 15, wherein the central controller isconfigured to set a voltage reference or a current reference at thesecond terminals of the inverter.
 17. The system of claim 16, whereinthe voltage conversion ratio for the energy storage device is based on:the charge fraction for the energy storage device, and the voltagereference or the current reference.
 18. The system of claim 17, whereinthe central controller is configured to adjust the voltage reference orthe current reference based on: one or more sets of operating conditionsof the power converter, wherein the one or more sets of operatingconditions comprise buck mode, boost mode, temperature, voltage,current, or duty cycle, and a predetermined compensation factor for theenergy storage device.
 19. The system of claim 18, wherein the centralcontroller is configured to adjust the voltage reference or the currentreference based on: multiple efficiency values that respectivelycorrespond to the one or more sets of operating conditions, wherein themultiple efficiency values are stored in a memory operatively attachedto the central controller.
 20. The system of claim 17, wherein the powerconverter is configured to transfer power from the energy storage deviceand to the electrical network, or from the electrical network and to theenergy storage device, with variable rates such that the energy storagedevice reaches a respective charged or discharged level at predeterminedend times.